YU1AW is a prolific designer! Watch this space for
additions at any time!

Dragan YU1AW, has kindly granted me
permission to present his RF power amplifier designs, even though he already has a web site
of his own! His home page
is on QSL.net!

Although my purpose is to present
information, or links to same, regarding Power Amplifiers which are designed and built around
"surplus" Russian tubes, I would be remiss in not stating that YU1AW is an RF engineer. Not
only does he design and build Power Amplifiers, he an excellent line of
high dynamic range preamplifier designs on his pages, as well! Please check these
at his
preamplifier web page.

YU1AW QRO: PAs using Russian
Power Triodes

Although thermal drift problems and physical size (too large in terms of electrical
wavelength) render it problematic for use at power output levels greater than about
250W on 23cm (links to or drawings of several designs are shown on this site), the
GI-7b is well
suited for use in multiple-tube amplifiers from HF through 70cm. Several successful
designs have been developed to use this inexpensive tube to produce respectable,
drift-free power output on these lower frequencies. The excellent YU1AW contributions
to the cadré of GI-7b PAs is presented here. Additionally, designs using the
GS-31B and
GS-35B are aslo shown.
The "Easy-Build" ("EB") PAs are Linear Amplifiers designed using HF
design techniques. The result is a family of 6m & 2m PAs which use standard inductors and
capacitors rather than tuned lines in the anode circuit.

YU1AW designs loaded on this site(*) indicates most recent additions or updates.

The GS-31B is the "little
brother" to the powerhouse GS-35.
The primary difference between the two is the lower anode dissipation of 1000W for the GS-31
vice 1500W for the GS-35. This is due to different design of the anode cooling structure
(both the physical interface with the detachable radiator, AND the type of radiator used with
the tube). Russian tube manuals also say the design life of the GS-31 is not less than 1000
hours of operation, while for the GS-35, 650 hours is claimed. There are no other significant
differences; these two tubes can be interchanged, electrically. The GS-31 is a good performer,
capable of high-duty-cycle output of 1200W at 2m. A single GS-35 can easily exceed 1500W
output on all bands through 432MHz.

DESIGN NOTES:

Input Circuit Q: There are at least three reasons
to design input circuits for a
relatively high Q and accept the compromises with respect to a lower Q of 3 or 4:First, higher input Q allow higher transformation ratio at input circuit. This is very
important because many drivers do not exhibit non-reactive 50 ohm output impedance. This
impedance, transformed via a length of coaxial cable connecting the driver to the PA, can
result in "wild" excursions of impedance far from 50 ohms. Even with input circuit Q = 20,
it is possible to exceed the range of acceptable impedances. If in tuning PA input it is
found that capacitors Ctk and Cck reach limits of their values (max or min capacity), change
length of coax cable between driver and PA to produce different impedance transformation and
retune. Repeat until input circuit tuning is successful.Second, if operating frequency is moved too far from plate circuit resonance, tube
damage can result if it is still possible to drive the PA to full power output. It is easy
to avoid this situation by making the bandwidth of the input circuit narrower than that of the
output circuit. This prevents driving the PA to full power output under off
frequency conditions.Third, driving voltage is rectified at the input to the active PA device (tube or transistor)
which acts like a diode. As a result "half of Fo" frequency is produced, which must be
suppressed. One of the best way to reject this "half Fo" is narrowing input circuit bandwidth
enough to decrease impedance of input circuit on Fo/2 frequency to some low value which
effectively shorts Fo/2 voltage to ground. This improves PA stability significantly!
Observation: Higher Q produce higher losses, but it is not very important because it only
increases drive power requirements by 1dB or so. Input circuit Q has to be low enough to allow
operation over an acceptable bandwidth without retuning the input circuit. Input circuits with
Q = 20 yield a wider band than it is necessary for normal VHF work but, as above, result in
bandwidth narrower than that of the plate circuit!

GI-7B Bias Voltage: Due to significant differences
between GI-7B tubes (new and
surplus), and because plate voltage can increase significantly from full load no load
condition with some power supplies, it is difficult to predict exactly what bias voltage to
use. Start with around 30V and adjust up or down to achieve an idle current of about 30-50 mA
per tube. Very precise value is not very important, but it IS important in multi-tube PA that
all tubes have same plate current (Ia)! Bias resulting in higher idle current gives higher
gain but less efficiency, and vice versa. The most important consideration in multi-tube
design is that all tubes have very similar maximum Ia! Measurement of air temperature for
each tube gives good information on dissipation and the relative Ia for each tube.

GI-7B Plate Voltage: The GI-7B can operate with up
to 2500VDC on the plate. This
voltage can, however, cause the tube to arc in a catastrophic manner (ruin the tube) if the PA
is too lightly loaded. It is best to limit plate voltage on this tube to about 2200VDC.